Analysis of the Decorator Pattern in Java Design Patterns

The decorator pattern is a structural design pattern that allows dynamically adding object functionality without modifying the original class. It is implemented through the collaboration of abstract components, concrete components, abstract decorators and concrete decorators, and can flexibly expand class functions to meet changing needs. In this example, milk and mocha decorators are added to Espresso for a total price of $2.29, demonstrating the power of the decorator pattern in dynamically modifying the behavior of objects.

Analysis of the Decorator Pattern in Java Design Patterns

Introduction

Decorator A pattern is a structural design pattern that allows functionality to be dynamically added to an object without modifying its base class. By using decorator objects, class functionality can be flexibly extended to meet changing needs.

Principle

The decorator pattern works in the following way:

1. Define an abstract component class that declares the public properties of the object that needs to be decorated interface.
2. Implement a concrete component class, which implements the interface of the abstract component class.
3. Define an abstract decorator class that defines an interface for additional functionality that can be attached to a concrete component.
4. Implement the concrete decorator class, which extends the abstract decorator class and implements specific functions.
5. Use the decorator pattern to enhance component functionality by wrapping specific decorators around specific components.

Code Example

// 抽象组件
interface Beverage {
    double cost();
}

// 具体组件
class Espresso implements Beverage {
    @Override
    public double cost() {
        return 1.99;
    }
}

// 抽象装饰器
abstract class CondimentDecorator implements Beverage {
    protected Beverage beverage;
    
    public CondimentDecorator(Beverage beverage) {
        this.beverage = beverage;
    }
}

// 具体装饰器
class Milk extends CondimentDecorator {
    public Milk(Beverage beverage) {
        super(beverage);
    }
    
    @Override
    public double cost() {
        return beverage.cost() + 0.10;
    }
}

// 具体装饰器
class Mocha extends CondimentDecorator {
    public Mocha(Beverage beverage) {
        super(beverage);
    }
    
    @Override
    public double cost() {
        return beverage.cost() + 0.20;
    }
}

// 实战案例
public class CoffeeShop {
    public static void main(String[] args) {
        Beverage espresso = new Espresso();
        
        // 添加牛奶和摩卡装饰器
        Beverage milkEspresso = new Milk(espresso);
        Beverage mochaMilkEspresso = new Mocha(milkEspresso);
        
        // 计算饮料总价
        double totalCost = mochaMilkEspresso.cost();
        
        System.out.println("Beverage cost: " + totalCost);
    }
}

Output:

Beverage cost: 2.29

Conclusion

The decorator pattern can be used to dynamically extend object functionality without modifying existing code. It provides a flexible and elegant way to meet changing needs while keeping the code maintainable and extensible.

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